1. Field of the Invention
This invention relates to hollow, elongated tubular solid oxide fuel cells, including cylindrical and Delta/triangular types, that can operate between 600° C. to 800° C., and supporting substrates that have a thin, cost effective, mechanically strong, and porous metal structure.
2. Description of the Prior Art
High temperature solid oxide ceramic electrolyte fuel cell (“SOFC”) electrochemical generator devices, which operate at about 1,000° C., are disclosed, for example, in Isenberg, U.S. Pat. No. 4,395,468 and Isenberg, U.S. Pat. No. 4,490,444. Such electrochemical generator devices comprise a plurality of elongated, typically annular, fuel cells which convert chemical energy into direct-current electrical energy. The fuel cells can be interconnected in series to provide a desired voltage and/or in parallel to provide a desired current capacity.
Each fuel cell typically includes an optional porous support tube of calcia stabilized zirconia. A porous annular air electrode or cathode generally surrounds the outer periphery of the support tube. The air electrode can be made with doped oxides of the perovskite family, such as, for example, lanthanum manganite) LaMnO3. A dense layer of gas-tight solid electrolyte, typically yttria stabilized zirconia (ZrO2), substantially surrounds the outer periphery of the air electrode. A porous fuel electrode or anode, typically of nickel-zirconia cermet or cobalt-zirconia cermet, substantially surrounds the outer periphery of the solid electrolyte. Both the solid electrolyte and the outer electrode, or, in this case the fuel electrode, are discontinuous to allow for inclusion of an electrically conductive interconnection material providing means to connect adjacent fuel cells. A selected segment of the air electrode is covered by the interconnection material. The interconnection material may comprise a doped lanthanum chromite (LaCrO3) film. The generally used dopant is Mg, although other dopants such as Ca and Sr have been suggested. The dopant serves to enhance the conductivity of the lanthanum chromite p-type conductor.
In this regard, referring to FIG. 1, a prior art tubular, electrochemical cell 10 is shown. The preferred configuration is based upon a high temperature solid oxide electrolyte fuel cell system, operating at about 1,000° C., wherein a flowing gaseous fuel, such as hydrogen, carbon monoxide or unreformed hydrocarbon gases, is directed over the outside of the cell, axially in the embodiment of FIG. 1 in the direction shown by fuel arrow 12. An oxidant, such as air or O2, is directed through the inside of the cell, as shown by oxidant arrow 14. Oxygen molecules pass through porous, thick ceramic support 22 and thick, porous electrically conductive air electrode structure 16 and are converted to oxygen ions which pass through a solid oxide ceramic electrolyte 18 at 1,000° C., to combine with the fuel at a fuel electrode 20. A discontinuous section of ceramic interconnection is shown as 26.
More recently, the calcia stabilized zirconia support has been eliminated by use of self-supporting air electrodes, as illustrated in U.S. Pat. No. 5,916,700 (Ruka et al.), having a thickness of 1.0 mm to 3.0 mm, made of doped, sintered lanthanum manganite, and the covering electrolyte is taught to be about 0.001 mm to 0.1 mm thick. These fuel cells operate at 1,000° C. with a gaseous fuel such as H2, CO or natural gas, and operation at that temperature excluded use of metal, or metal alloys.
The use of self-supporting, ceramic air electrodes was brought about because the ceramic calcia stabilized zirconia support structures added 1.0 mm to 2.0 mm thickness to the structure, required up to 14 hours sintering at 1,650° C. and were a compromise between mechanical strength and O2 gas diffusion. To allow thinner ceramic support structures, Rossing et al. (U.S. Pat. No. 4,598,028) added 3 wt. % to 45 wt. % thermally stable oxide fibers to the ceramic powder to provide an interlocked fiber/powder structure. The resulting thickness was from 0.5 mm to 2.0 mm, and the separate support still operated in fuel cells operating at 1,000° C. The use of fibers in the separate ceramic support, however, while possibly reducing ceramic support structures by about 0.5 mm, required additional steps and added cost.
Other tubular, elongated, hollow fuel cell structures are described by Isenberg, in U.S. Pat. No. 4,728,584—“corrugated design” and by Greiner et al.—“triangular”, “quadrilateral”, “oval”, “stepped triangle” and a “meander”; all herein considered as hollow elongated tubes. A hollow elongated tubular geometry of particular interest has the geometric form of a number of integrally connected elements of triangular or “delta” like cross section, see FIG. 3 of the drawings. These triangular, elongated, hollow cells have been referred to in some instances as Delta X cells where Delta is derived from the triangular shape of the elements and X is the number of elements. These type cells are described for example in basic, Argonne Labs U.S. Pat. No. 4,476,198; and also in U.S. Pat. No. 4,874,678; U.S. Patent Application Publication 2008/0003478 A1, and International Publication No. WO 02/37589 A2 (Ackerman et al., Reichner; Greiner et al., and Thomas et al., respectively).
Generally, in newer triangular, tubular, elongated, hollow cross-section, so called Delta X cells, the resulting overall cross section has a flat face on the interconnection side and a multi-faceted triangular face on the anode side. Air flows within the internal discrete passages of triangular shapes where, at the end of the cell, the air can reverse flow to diffuse through the air electrode if air feed tubes are used. A basic encyclopedic publication by N. Q. Minh, in “Ceramic Fuel Cells”, J. Am. Ceramic Soc., 76 [3] 563-588, 1993 describes in detail a variety of fuel cell designs, including the tubular and triangular and other types, as well as materials used and accompanying electrochemical reactions.
Nguyen Q. Minh also describes seal-less, tubular SOFCs, which operate at about 1,000° C. so that the solid oxide electrolyte is effective, describes SOFC supports as of 1993. Those separate SOFC supports were extruded CaO stabilized zirconia of 35% porosity, having a 1.0 mm to 1.5 mm thickness and a 36 cm to 1 meter length overlaid with a 35% porous 1.4 mm thick strontium doped LaMnO3 air electrode and a 40 micrometer (0.04 mm) thick gas tight Y2O3 stabilized ZrO2 electrolyte.
In another type fuel cell design, Jacobson et al. (U.S. Pat. No. 7,232,626) teaches two main SOFC's; tubular and planar, dramatically different, each having advantages and disadvantages. Jacobson et al. recognized the fact that exotic powders such as 99% pure zirconia etc. cost $30 to $60/lb. while, for example, stainless steel costs $2/lb., so that support and interconnection plates of metal sheet, sintered powder or wire mesh of 0.5 mm to 0.75 mm (500 micrometer to 750 micrometer) could provide reduced manufacturing costs. One of the major disadvantages of the Jacobson et al. SOFC design, however, includes poor bonding strength caused by the formation of oxide scale on the surface of metal substrates, which leads to a high contact resistance and performance degradation during operation.
The cost of ceramic air electrode supported, seal-less, tubular SOFCs is the major hurdle to commercialization. Whether the cost can be lowered to the level comparable to the existing power generation technology and acceptable for market entry determines critically, the success of the SOFC technology. Therefore, cost reduction in any area or component is a critical path for the current product development, especially if cost reduction in one component leads to further cost reduction in other components as a consequence.
High product cost is associated with the high operating temperature of a SOFC. If a SOFC operates at high temperatures, about 1,000° C., then: 1) the cell and module materials used are limited to the expensive high-purity and high-strength category; 2) the cell design has to adopt one-end closed tubular geometry for the consideration of effective internal heat exchange. Although use of ceramic air feed tubes to deliver the needed air and recuperate the heat given off from the stack avoids using expensive exotic external heat exchangers, the high-purity Al2O3 air feed tubes are very costly; 3) the thermal management of the stack temperature requires a convoluted airflow to achieve a uniform temperature distribution, not to mention that it takes much longer time to startup and shutdown; 4) a pre-reformer separate from the stack is needed to reform incoming hydrocarbon fuels into simple fuels; and 4) the stack electrical efficiency is compromised by a lowered intrinsic thermodynamic efficiency and higher fuel losses at high temperatures.
In contrast, “intermediate-temperature” SOFCs, about 600° C. to 800° C., avoid every drawback aforementioned for “high temperature”—1,000° C.—SOFCs. In addition, the electrical efficiency is particularly boosted by the intrinsic thermodynamic fuel conversion efficiency, lower mixed-conduction related fuel loss and current collector related power loss that is critically important for low-voltage and high-current power generation.
What is needed is a new design of low-cost SOFC for applications at low or intermediate temperatures, where air electrode thickness and thus ceramic materials costs are reduced, yet where a strong porous support of some sort will still be present.
It is an object of this invention to provide a low cost intermediate operational SOFC having a very porous but also a very strong and very thin support on which other functional layers, that are in general costly, can exist in the SOFC in the form of very thin films.
It is another object of this invention to provide a strongly bonded and low contact resistance ceramic layer that enables single SOFCs to be combined into SOFC bundles (plurality of SOFC's), and to operate as open-end ‘once-through’ fuel cells.